The present invention relates generally to signal processing systems and more particularly to systems for generating digital, compressed domain signals such as ATSC and DVB compliant signals.
As the popularity of digital broadcasting systems grows so does the demand for reliable, efficient and affordable digital transmission systems. A particular application currently gaining widespread acceptance and support is digital television. The ATSC (Advanced Television Systems Committee) DTV (Digital Television) standard describes a system adapted to transmit high-quality video, audio and ancillary data over a single channel. The present invention will be discussed as it relates to the ATSC DTV standard for sake of explanation, however it should be understood the present invention is equally applicable to other digital broadcasting systems and applications as well. An alternative approach to which the present invention is equally applicable, for example, is the DVB (Digital Video Broadcasting) standard for example.
As is well known, the ATSC broadcast system includes three general subsystems: source coding and compression, service multiplex and transport, and transmission. Basically, the first subsystem, source coding and compression, deals with bit rate reduction for the video, audio and ancillary digital data streams. The second subsystem, service multiplex and transport, deals with dividing each digital stream into xe2x80x9cpacketsxe2x80x9d and multiplexing the video stream packets, audio stream packets and ancillary digital data stream packets into a single transport stream. The ATSC standard employs the Motion Pictures Experts Group MPEG-2 transport stream syntax for packetization and multiplex of the video, audio and ancillary digital data. Finally, the third subsystem, transmission, deals with channel coding and modulation.
The present invention deals particularly with the second subsystem, service multiplex and transport. For a more detailed explanation of the ATSC standard, the reader is referred to xe2x80x9cATSC Digital Television Standardxe2x80x9d, ATSC Standard A/53 (1995) and xe2x80x9cGuide to the Use of the ATSC Digital Television Standardxe2x80x9d, Doc. A/54 (1995) both published by the Advanced Television Standards Committee, Washington D.C., the entire disclosures of which are hereby incorporated by reference as if being set forth herein in their respective entireties.
One approach to creating ATSC transport packets from compressed video, audio and data streams is illustrated in FIG. 1. Payload data, which in the embodiment of FIG. 1 takes the form of compressed video data at input 12, compressed audio data at input 14 and compressed ancillary data at input 16 from Packetized Elementary Stream (xe2x80x9cPESxe2x80x9d) encoders 10 are sent to the Transport Stream Encoder (xe2x80x9cTSExe2x80x9d) 20 where 188 byte transport packets at output 22 are formed therefrom. Each 188 byte transport packet at output 22 can include a fixed-length link layer, a variable-length adaptation layer and a payload of data as is well understood. As is also known, the fixed link layer is used for packet synchronization, packet identification, error correction and conditional access. The adaptation layer also serves to provide synchronization, as well as to enable random entry into the data stream and local programming insertion. As is understood by those possessing ordinary skill in the art, unlike conventional analog NTSC television signals which utilize the concept of synch pulses whereby a clock rate can be directly derived from each picture itself, the amount of data for each picture in a compressed digital system is variable. Hence, a loss of synchronization in a compressed domain digital system can readily lead to buffer over- or underflow. To mitigate this possibility, timing or synchronization data is transmitted in select ones of the output transport packets, 22 referred to as Program Clock Reference (xe2x80x9cPCRxe2x80x9d) packets.
Still referring to FIG. 1, the approach illustrated therein is to supply a reference frequency 40 to a clock generator 30. The clock generator 30 supplies timing to the sampling mechanisms in the encoders 10, 20 and to the timing packet generation system PCR GEN 50, to create a PCR packet for synchronizing signal receiving decoders. Because internal buffers 21, 61 within the TSE 20 and output interface 60 respectively are typically utilized, and to avoid variable latencies associated with such buffering, the PCR packet is multiplexed at the output interface 60 instead of within the TSE 20 where the other packets are formed. Thus, the system requires a PCR generator 50 at the output as is illustrated in FIG. 1. An independent output byte clock 70 is used to transfer the transport packet bytes from the output interface 60 to the transmission equipment using a conventional driver stage 65, e.g. to the third subsystem.
However, such a configuration yields an undesirably complex device as multistage counters (42 stages for example) are typically required for the PCR generator.
It is an object of the present invention to simplify the method and system of transport packet generation and PCR packet insertion by moving the functionality of PCR packet formation and insertion into the TSE 20.
A device and method for utilizing a single clock signal to generate a digital data stream signal for transmission in a compressed domain transmission system. The device includes a plurality of packetized elementary stream encoders electronically coupled to a transport stream encoder electronically coupled to an output interface adapted to generate the digital data stream signal. The method includes: operating each of the packetized elementary stream encoders responsively to the single clock sequence to generate a plurality of packetized elementary stream signals; operating the transport stream encoder responsively to the single clock signal to form a transport stream signal from the plurality of packetized elementary stream signals, wherein the transport stream signal includes a plurality of data packets each formed using the transport stream encoder and select ones of the plurality of data packets formed by the transport stream encoder include synchronization data; and, operating the output interface responsively to the single clock reference to output the digital data stream signal in compliance with a predefined manner.